1. Field of the Invention
The present invention relates to a method of reproducing an original. More specifically the present invention relates to a method of determining a transformation for producing colour separations of an original that are suitable for reproducing an original according to non-standard reproduction rules.
2. Description of the Prior Art
In a graphic arts reproduction process a full colour reproduction is obtained by printing with at least three basic colour inks separately, one after the other in perfect register.
It is possible to reproduce most full colour copies by the use of three primary colour inks cyan, magenta and yellow. In most cases an extra printing cycle with black ink is performed to obtain good printing quality.
The printing plates for printing in register with each of said primary colours are produced by means of a set of primary colour separations of the full colour original that is to be reproduced.
The colour separations made from a colour original are commonly obtained by means of a reproduction camera or by means of an electronic colour separator, also called a colour scanner.
In such a colour scanner, a light-beam is used to scan an original and to measure the density in each minute area of the original through each of a set of primary colour filters. The density values are converted into electrical signals and are used for controlling a light source for exposing photographic film in order to produce colour separations.
An electronic colour scanner is useful for obtaining colour separations much faster than with the aid of a reproduction camera. However, it is a rather complex apparatus and demands a lot of experience of the operator. The scanner must be adjusted according to the specific reproduction process that will be used, the characteristics of an original and the individual desires of a client.
For example, corrections may have to be implemented for matching the colour characteristics and density range of e.g. a colour film original, the reproducing material and the printing inks.
So, due to the various types of originals that are to be reproduced, the interpretation of an original that is to be performed by the scanner operator, the knowledge of the reproducing process and the apparatus capabilities, the process of obtaining colour separations is made rather complex and dependent on the operator's skill and experience.
Even through manipulation of the scanner by highly skilled operators, mutual differences still exist between reproductions made by different operators due to their different personal interpretation of the coloured originals.
An objective technique of interpreting an original and adjusting an electronic scanner in accordance with such interpretation or in accordance with the desired reproduction result would therefore be very helpful.
A prior art proces for classifying an original to be reproduced and for adjusting a scanner set up has been described in U.S. Pat. No. 4,472,736.
According to this classifying process a number of "standard reproductions" are selected from a large number of high quality reproductions. Standard density distribution patterns of these standard reproductions are obtained and stored. The density distribution pattern of an original to be reproduced is then compared with the density distributions of said standard reproductions. One of the standard density distribution patterns of the standard reproductions which is closest to that of the original to be reproduced is then selected and the original to be reproduced is classified in the class of said selected standard reproduction.
A colour separation tone curve is implemented on a scanner by determining set values for the adjustment controls of the scanner.
According to this prior art method, these set values are determined so as to enable faithful reproduction of the high quality standard originals on the scanner.
Then a standardized pattern of the original to be reproduced, comprising at least the cumulative density values with respect to the entire density range, is formed. This pattern is compared with the patterns in respect of the standard reproductions and a pattern that is similar to that of the standard reproduction is selected.
The scanner set up values corresponding to the standard reproduction having the selected density distribution pattern is used for reproduction of the original.
Experience and practice have established that the originals to be reproduced are far from being "high quality" originals and very often the client wishes to have these originals reproduced as "high quality" reproductions or to have made specific modifications to the reproduction which are not part of a standard reproduction process aiming at a "true, faithful" reproduction.
The prior art method hereinbefore described does not provide a procedure for obtaining a high quality reproduction of an original (more specifically a corrected, modified reproduction) which is not a "high quality" original.